basic op amp circuits

Engr.Tehseen Ahsan

Lecturer, Electrical Engineering Department

EE-307 Electronic Systems Design

HITEC University Taxila Cantt, Pakistan

Basic Op-Amp Circuits

13-1 Comparators

Operational amplifiers are often used as comparators to

compare the amplitude of one voltage with another.

In this application, the op-amp is used in open-loop

configuration, with the input voltage on one input and a

reference voltage on the other.

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13-1 Comparators Continue…

Zero-Level Detection

One application of an op-amp used as a comparator is to

determine when an input voltage exceeds a certain level.

Figure 13.1 (a) next slide shows a zero-level detector. Notice

that the inverting (-) input is grounded to produce a zero-level

and that the input signal voltage is applied to the non-inverting

(+) input.

The input voltage Vin at the non-inverting (+) input is compared

with a reference voltage VREF at the inverting input (VREF = 0V).

Since VREF = 0V, this is called a zero-level detector.

Because of the high-open loop voltage gain, a very small

difference voltage Vd between the two inputs drives the

amplifier into saturation ( non-linear region).

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13-1 Comparators Continue… Zero-Level Detection

Figure 13-1 (b) shows the result of a sinusoidal input voltage

applied to the non-inverting (+) input of the zero-level

detector. When the sine wave is positive, the output is at its

maximum positive level. When the sine wave crosses 0, the

amplifier is driven to its opposite state and the output goes to

its maximum negative level.

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13-1 Comparators Continue… Zero-Level Detection

When Vin> VREF ( Sine wave is positive)

Vd = Vin- VREF

Vd > 0V

Vout = + Vout(max)

When Vin<VREF ( Sine wave is negative)

Vd = Vin- VREF

Vd < 0V

Vout = - Vout(max)

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Nonzero-Level Detection

A more practical arrangement is shown in figure 13-2 (b) next

slide using a voltage divider to set the reference voltage VREF as

Where +V is the positive op-amp dc supply voltage.

The circuit in figure 13-2 (c) next slide uses a zener diode to

set the reference voltage (VREF = VZ).

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Nonzero-Level Detection

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Most practically used

13-1 Comparators Continue… Nonzero-Level Detection

Voltage – divider Reference ( figure 13-2 (b) )

When Vin> VREF

Vd = Vin- VREF

Vd > 0V

Vout = + Vout(max)

When Vin<VREF

Vd = Vin- VREF

Vd < 0V

Vout = - Vout(max)

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13-1 Comparators Continue… Nonzero-Level Detection

Zener diode sets reference voltage ( figure 13-2 (c) )

When Vin> VZ

Vd = Vin- VZ

Vd > 0V

Vout = + Vout(max)

When Vin< VZ

Vd = Vin- VZ

Vd < 0V

Vout = - Vout(max)

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13-1 Comparators Continue… Effects of Input Noise on Comparator Operation

In many practical applications, noise (unwanted voltage

fluctuations) appears on the input line.

This noise becomes superimposed on the input voltage as

shown in figure 13-5 for the case of a sine wave and can cause a

comparator to erratically switch output states.

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In order to understand the potential effects of noise voltage,

consider a low-frequency sinusoidal voltage applied to the non

inverting (+) input of an op-amp comparator used as a zero-

level detector as shown in figure 13-6 (a) next slide.

Figure 13-6 (b) next slide shows the input sine wave plus noise

and the resulting output.

As we can see when the sine wave approaches 0, the fluctuations

due to noise cause the total input to vary above and below 0

several times, thus producing an erratic output voltage.

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13-1 Comparators Continue… Reducing Noise Effects with Hysteresis

An erratic output voltage caused by noise on the input occurs

because the output voltage switches states several times at the

same input voltage level (output voltage switches states several

times at + ve half cycle and same for –ve half cycle).

In order to make the comparator less sensitive to noise, a

technique called hysteresis with positive feedback can be

used.

Hysteresis

There exists a higher reference level i.e., + VREF when input

goes from lower to higher value.

There exists a lower reference level i.e., - VREF when the input

goes from higher to lower value.

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The two reference levels are referred to as the upper trigger point

(UTP) and lower trigger point (LTP).

This two-level hysteresis is established with a positive feedback

arrangement as shown in figure 13-7.

Note that the noninverting (+) input is connected to the a resistive

voltage divider such that a portion of the output voltage is fed back to

the input. The input signal is applied to inverting input (-) input in

this case.

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This configuration is also called Schmitt Trigger


The basic operation of the comparator with hysteresis is illustrated in

figure 13-8 next slide(s). Assume that the output voltage is at its

positive maximum + Vout(max). The voltage fed back to the non inverting

input is VUTP and is expressed as

When Vin exceeds VUTP , the output voltage drops to its negative

maximum, -Vout(max) as shown in part (a). Now the voltage fed back to

the non inverting input is VLTP and is expressed as

A comparator with hysteresis is also called Schmitt trigger. The

amount of hysteresis can be found as

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13-1 Comparators Continue… Output Bounding

In some applications, it is necessary to limit the output voltage levels

of a comparator than that provided by the saturated op-amp.

A single zener diode can be used as shown in figure13-10 to limit the

output voltage to the zener voltage in one direction and to the

forward drop in other. The process of limiting the output is called

bounding.

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13-1 Comparators Continue… Positive Value Output Bounding

When anode is connected to a negative terminal.

In positive half cycle of output voltage, the zener diode gets reverse-

biased and the limits the output voltage to the zener voltage i.e., +VZ

In negative half cycle of output voltage, the zener diode gets forward

biased and behaves as a normal conventional diode with a drop of 0.7

V across it ( -0.7 V) . It is shown in figure 13-11 (a) below

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13-1 Comparators Continue… Negative Value Output Bounding

When cathode is connected to a negative terminal.

In positive half cycle of output voltage, the zener diode gets forward

biased and behaves as a normal conventional diode with a drop of 0.7

V across it ( + 0.7 V) .

In negative half cycle of output voltage, the zener diode gets reverse-

biased and the limits the output voltage to the zener voltage i.e., it is

shown in figure 13-11 (b) below

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13-1 Comparators Continue… Double Bounded Comparator

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13-2 Summing Amplifiers

Summing Amplifier with Unity Gain

A summing amplifier has two or more inputs and its output

voltage is proportional to the negative of the algebraic sum of

its input voltages.

A two-input summing amplifier is shown in figure 13-20, but

any number of inputs can be used.

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13-2 Summing Amplifiers Continue…


The operations of the circuit and derivation of the output

expression are as follows:

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Summing Amplifier with Gain Greater Than Unity

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Averaging Amplifier

Averaging amplifier is a variation of summing amplifier.

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Scaling Adder

Scaling Adder is also a variation of summing amplifier.

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13-3 Integrators and Differentiators

The Op-Amp Integrator

An ideal integrator is shown in figure 13-31. Notice that the feedback

element is a capacitor that forms and RC circuit with the input

resistor.

Practical integrators often have an additional resistor Rf in parallel

with the feedback capacitor to prevent saturation. However we will

consider the ideal integrator for the purpose of our analysis as it does

not affect the basic operation.

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13-3 Integrators and Differentiators Continue…


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13-3 Integrators and Differentiators Continue… The Op-Amp Integrator

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The Op-Amp Differentiator

An ideal differentiator is shown in figure 13-37. Notice how the

placement of the capacitor and resistor differ from the integrator. The

capacitor is now the input element and resistor is the feedback

element. A differentiator produces and output that is proportional to

the rate of change of the input voltage.

Practical differentiators may include a series resistor Rin to reduce

high frequency noise.

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